This invention relates to an active filter circuit, and is suitable for an application particularly to a low-pass filter circuit.
A variety of filter circuits have hitherto been constructed by combinations of passive elements such as capacitors, resistances, etc. and active elements such as transistors, etc., because an inductor can not be mounted on an integrated circuit (IC).
For instance, as illustrated in FIG. 1, one terminal of a capacitor C is connected to a connecting node P0 between a resistance R and an output terminal, while the other terminal thereof is grounded, thus constructing a primary low-pass filter.
By the way, a cut-off frequency f of this low-pass filter can be expressed by the following formula: ##EQU1##
There exist, however, scatters of approximately .+-.15-20 [%] and .+-.6-15 [%] in terms of accuracy in the IC. Further, the resistance R exhibits a temperature characteristic to have an amount of variation of approximately 3000 [ppm/.degree.C.], i.e., nearly 30 [%] at 100 [.degree.C.]. This causes a problem wherein the cut-off frequency is not constant.
Then, low-pass filters illustrated in FIGS. 2 and 3 have been proposed in order to make the cut-off frequency f constant irrespective of fluctuations in resistance value of the above-mentioned resistance R or in capacitance of the capacitor C.
Voltage-controlled, to be specific, are voltage characteristics (shown in FIG. 4) of a junction capacitance (hereinafter referred to as a voltage variable capacitance) composed of a diode or voltage characteristics (shown in FIG. 5) of a variable resistance value R.sub.j of a depletion type MOS transistor (hereinafter abbreviated to D-MOS (depletion metal oxide semiconductor)). A product C.multidot.R of the capacitance and the resistance value is made constant on the whole, whereby the cut-off frequency f becomes constant.
The D-MOS herein exhibits such a nature that a drain current I.sub.D flows simply by applying a drain-source voltage V.sub.D and reduces as a gate voltage V.sub.G increases in the negative direction.
As depicted in FIG. 2, the scatters caused in the resistance R and in the capacitance C are compensated by making the voltage variable capacitance C.sub.j variable in order to control the value C.multidot.R to a constant value. In this case, the voltage variable capacitance C.sub.j is required to be variable in a range of +20 [%] or greater. Hence, a region of 0-2 [V] which has the largest voltage dependency has to be used as an inter junction reverse bias voltage V.sub.R (&gt;0) (FIG. 4).
In the case of making the voltage variable capacitance C.sub.j variable by using this region, however, because of utilizing a non-linear region, an input signal level is as small as 0.2 [Vp-p] in order that a distortion of the output signal falls within 1 [%].
Besides, the input signal level can not be set large, and hence there arises such a problem that a deterioration in signal-to-noise ratio (hereinafter referred to as an S/N ratio) is inevitable.
On the other hand, as illustrated in FIG. 3, the scatters produced in the resistance R and in the capacitance C are compensated by making the variable resistance value R.sub.j variable to control the value of C.multidot.R to a constant value. In this case also, because of the necessity for largely varying the variable resistance value R.sub.j, it is required that the non-linear region be utilized. This conduces to a problem wherein the input signal level can not be set large (FIG. 5).
For this purpose, a low-pass filter 1 (FIG. 6) employing a transconductance variable type differential amplifier circuit has been proposed so that the input signal level can be set large.
The low-pass filter 1 consisting of a 2-stage low-pass filter is constructed such that an input signal V.sub.in passes through first and second operational amplifiers 2 and 3 constituting a current mirror type differential amplifier circuit and is thereafter output as an output signal V.sub.out from a buffer 4.
A differential output V1 of the first operational amplifier 2 is herein input to a non-inverting input terminal of the second operational amplifier 3. A capacitor C1 is connected to a connecting node P1 between the output terminal of the amplifier circuit 2 and the non-inverting input terminal of the amplifier circuit 3.
A differential output V2 of the operational amplifier 3 is supplied to the buffer 4. At the same time, a capacitor C2 is connected to a connecting node P2 between the output terminal of the second operational amplifier 3 and the input terminal of the buffer 4.
The output signals V.sub.out are respectively fed back to inverting input terminals of the first and second operational amplifiers 2 and 3, thereby amplifying a difference in electric potentials between the input signal V.sub.in and the output signal V.sub.out, and between the output signal V1 of the first operational amplifier 2 and the output signal V.sub.out.
The capacitors C1 and C2 are herein defined as non-voltage-dependent capacitors composed of MIS (Metal Insulator Semiconductor) or MOS (Metal Oxide Semiconductor) capacitors so connected that the capacitances thereof do not fluctuate due to parasitic capacitances which fluctuate depending on the voltage while being parasitic to the first and second operational amplifiers 2 and 3. The capacitances are selected larger by one order than the parasitic capacitance (i.e., 10 [pF] for 1 [pF]).
However, for instance, a size of 100.times.100 [.mu.m] is needed for taking large capacitances of the capacitors C1 and C2 formed in the IC. The IC itself has to increase in size for the transistor of 15.times.20 [.mu.m].
Additionally, when using a built-in capacitance C and a transconductance g.sub.m expressed by the following formula: ##EQU2## The cut-off frequency f of the low-pass filter 1 is expressed by the following formula: ##EQU3## It is, however, required that the transconductance g.sub.m be increased in proportion to the capacitance C to keep constant a value of the cut-off frequency f. A current I.sub.x supplied to the operational amplifiers 2 and 3 has to be set large.
For this reason, in the case of the low-pass filter composed of the transconductance variable type differential amplifier circuit, a dynamic range of the input signals expands. On the other hand, the chip is enlarged because of increments in areas of the intra-IC capacitors C1 and C2. A problem is caused, wherein the consumed electric power supplied to the IC chip also augments.